error code

hi all, i own a 02 camry xle v6 from new only 21k, its been troublr free until yesterday. check engine light came on, had auto zone read for code, one code p1135 sensor a/f bank1 sensor. called stealer, guy said car has 4 sensor will need to check each to see , that code says bank 1 sensor 1, does anyone had this code and if they tried to change the sensor themself. car runs smooth, but don't like to see this light all the time. seems early to have a sensor go bad at 21k. thanks in advance

af sensor closes to the firewall is bank 1 sensor 1.